Polymeric foams of many different chemical compositions have found widespread industrial and household applications. In particular, open-cell foams based primarily on polyurethane chemistry, can be formulated to show outstanding resiliency and cushioning properties. Polyurethane foams can be produced rapidly and inexpensively in a variety of sizes and complex shapes. Consequently, for cushions, mattresses, and pads, polyurethane foams currently dominate the upholstery and bedding industry, replacing the older, more expensive, and heavier foamed rubber (latex foam). Seats in automobiles, buses, and aircraft are now made almost exclusively from polyurethane foam. The development of foam-in-place techniques has extended the use of polyurethane foams as impact-absorbing materials in packaging and shipping containers. Foam-in-place techniques achieve rapid, in situ generation of polyurethane foam by mixing two liquid components, which react rapidly and exothermally to produce a polymer network in a variety of shapes, suitable for packaging. Foaming is produced by the flashing of low-boiling additives (foaming agents) and/or by the generation of carbon dioxide when water is present in the reaction system. In the packaging of fragile items, such as electronic equipment ranging from small components to televisions and computers, foamed-in-place polyurethanes are rapidly becoming the materials of choice. Other uses of polyurethane foams include thermal insulation and flotation (closed-cell foams).
In addition to polyurethane, foams based on polystyrene (styrofoam), polyethylene, plasticized polyvinyl chloride, and other polymers are used extensively in industrial and household applications. However, such prior-art polymer foams, especially those based on polyurethanes, have a very serious drawback: they are highly flammable. When exposed to flame they ignite easily and burn rapidly with the evolution of copious black smoke, highly toxic gases, and flaming liquids. Consequently the use of these materials has repeatedly caused (or contributed to) serious injury and loss of life in building, automobile, aircraft, and spacecraft cabin fires. Another limitation of prior art polymer foams is their low thermal stability: these materials, even in the absence of flame, decompose at temperatures above 200.degree. C., often producing toxic and/or flammable byproducts.
Extensive efforts to reduce the flammability of current polymer foams have met with limited success. The incorporation of known fire retardants as additives to the foam formulations or as chemically-bonded abducts to the polymer networks has invariably entailed deterioration of various desirable foam properties (increase in foam density, decrease in flexibility and/or strength) with only modest reduction in flammability.
The flammability of foams may be measured and compared in terms of a Limiting Oxygen Index (LOI). Limiting Oxygen Index (LOI) is a measure of the minimum concentration of oxygen (in a controlled oxygen-nitrogen atmosphere) that is necessary to support a flame in the test material for at least 3 minutes under specified test conditions (ASTM D2863 Test Standard). In general, since air contains 21% oxygen, materials with LOI values of or below 21% are considered highly flammable; to be considered "fire-retardant" a material must have LOI values in excess of 27.
Most "fire-retardant" polyurethane formulations show an increase of only 8-10 points in their Limiting Oxygen Index (LOI) values; 27-32 for fire-retardant polyurethanes compared to 16-22 for the basic systems. This lack of success in formulating nonflammable polyurethane foams is due to the low thermal stability of the polyurethane structures, which decompose at temperatures of about 200.degree. C., producing flammable liquids and toxic gases. Incorporation into the polyurethane molecular chains of thermally stable and/or fire retardant structures, such as isocyanurate rings, aromatic diols, and phosphorus, has also shown limited success in enhancing nonflammability (due to thermal decomposition of the urethane linkages) while considerably reducing the flexibility of the modified polyurethane foams. Other prior art foams, such as those based on polyvinyl chloride, while inherently nonflammable due to the presence of halogen, readily decompose upon heating with the release of the highly toxic and corrosive hydrogen chloride gas; consequently they are quite hazardous in fire situations.
The only current commercial foams that are inherently nonflammable are polyimide foams. These are formed by the expansion of poly(benzophenonetetracarboxylic imide) networks by dielectric heating, usually in microwave ovens. Foaming results from the evolution of water and/or other volatile by-products in the reaction. Foams with varying degrees of flexibility can be produced by varying the polyimide backbone structure. These foams have outstanding thermal stability and inherent nonflammability. They are nonigniting in air, showing LOI values of 39-43. In contact with an open flame these foams shrink and char with little smoke or toxic gas emission. Polyimide foams are marketed by the Imi-Tech Corporation under the trade name of "Solimide".RTM..
However, the current polyimide foams have several drawbacks. The cushioning properties of polymide foams are not as good as those of polyurethanes, in the sense that polyimide cushions are much less comfortable to sit or lie on; consequently, in applications where fire retardant foams are required (such as aircraft seats) polyimide foams are used as envelopes, enclosing polyurethane foam cores. Polymide foams cannot easily be foamed in place: they are available only in the form of block or slab stock, which, for packaging applications, must be individually cut and shaped, a very laborious and wasteful process. Although a method for in situ production of polyimide foams at ambient temperatures by the addition of up to 50% furfuryl ether to the system has been described in U.S. Pat. No. 4,184,201 (issued Jan. 15, 1980) this process is very difficult to control, and the use of the highly flammable furfuryl ether significantly reduces the flame retardency of the resulting foam. Consequently, this foam-in-place process has found little commercial application for production of a polyimide foam. Current methods of polyimide synthesis require expensive raw materials, and the foam production processes are slow and energy-consuming. Consequently polyimide foams sell for prices (currently about $80-90/kg) which are 20-30 times higher than the prices of most polyurethane foams.
It would be highly desirable to have new foam systems which would possess the advantages of polyurethane foams (low cost, easy to foam in place, good flexibility and resilience) as well as the nonflammability and thermal stability of the polyimide foams.